[Xen-users] Bad vmexit (reason 0x31) with HVMMEM_access_w/x

[Tamas Lengyel <tamas.lengyel@xxxxxxxxxxxx>]

Hi,
while playing with tools/tests/xen-access I came across domU crashes
when I set a page permission to either HVMMEM_access_w or
HVMMEM_access_wx and the domU tries to read from the page. I would
expect it to generate an event that is delivered to xen-access, but
that is not the case. All other HVMMEM_access flags work properly and
deliver the event, including read events as well. It seems to me that
making a page non-readable for the guest but keeping it writable is
what causing the issue but not sure where exactly. The vmexit is
caused by a EXIT_REASON_EPT_MISCONFIG which is unhandled in vmx.c.

xl dmesg:

(XEN) vmx.c:2847:d199 Bad vmexit (reason 0x31)
(XEN) domain_crash called from vmx.c:2848
(XEN) Domain 199 (vcpu#0) crashed on cpu#3:
(XEN) ----[ Xen-4.3.0  x86_64  debug=n  Tainted:    C ]----
(XEN) CPU:    3
(XEN) RIP:    0033:[<00007f7d4a0e2a48>]
(XEN) RFLAGS: 0000000000010207   CONTEXT: hvm guest
(XEN) rax: 0000000000000001   rbx: 00007f7d4a3ae240   rcx: 00007f7d4a0e2a40
(XEN) rdx: 0000000000000400   rsi: 00007f7d4a5d0000   rdi: 0000000000000000
(XEN) rbp: 00007f7d4a3ae160   rsp: 00007fff8c1197f8   r8:  00007f7d4a5ca700
(XEN) r9:  0000000000000000   r10: 0000000000000022   r11: 0000000000000246
(XEN) r12: 0000000000400540   r13: 00007fff8c11a960   r14: 0000000000000000
(XEN) r15: 0000000000000000   cr0: 0000000080050033   cr4: 00000000000406f0
(XEN) cr3: 0000000019b7a000   cr2: 00007f7d4a5d0000
(XEN) ds: 0000   es: 0000   fs: 0000   gs: 0000   ss: 002b   cs: 0033
(XEN) hap.c:265: d199 failed to allocate from HAP
pool<G><2>mem_event.c:518:d0 Ignoring memory event op on dying domain
199

I'm running Xen 4.3 and the issue was present on both an i7-2600 and
i3-2310M CPU.

Can anyone provide some insight into what might be going on here?

Thanks,
Tamas

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